The present invention generally pertains to transducer assemblies, such as a sensor assembly that enables communication to an external device from a transducer, and is particularly directed to the application of a sensing transducer to a subject area. As used herein the term “subject area” means the scalp, the chest or any other region of a human body. The state of a particular property of the selected subject area, such as bioelectrical potential, that is sensed by the transducer is communicated by the transducer assembly to an external device.
For EEG (electroencephalograph) applications, in which the sensing transducers are electrodes, sensor assemblies are used to apply electrodes to a subject area to enable the sensing of bioelectric potentials. For some subject areas, sensor assemblies that include so-called wet electrodes, which adhere to the body, are preferred. Conventional wet-electrode sensor assemblies include an Ag/AgCl disc (or a disc of some other conductive material). A wet conductive gel is used to establish an electrical connection through any hair between the subject area and the Ag/AgCl disc. Electrical potentials on the surface of the subject area are coupled via the gel to the Ag/AgCl disc and into an electronic amplifier. A standard wet electrode sensor assembly provides a secure, low-impedance electrical connection between the subject area and a recording instrument, and thereby ensures high quality signal sensing. However, the use of electrolytic gels in combination with the need for skin preparation is often time consuming for the user to set up, and irritating and uncomfortable for the subject. Alternatively, dry electrodes, which are designed to push through the hair to directly contact the scalp and do not require conductive gels or scalp preparation, have been explored as alternatives to wet electrodes.
In practice, dry electrodes suffer from numerous usability issues. Although acquiring signals on bare skin (e.g., forehead) is relatively straightforward, most EEG setups also require electrodes on areas of the head covered by hair. Patches of hair, depending on thickness, are often difficult to reliably penetrate and they block the sensor from physically reaching the skin. The presence of hair has made it challenging to build a dry EEG system that can be easily and rapidly donned by a subject with minimal assistance or adjustment.
Known prior art dry EEG electrodes typically utilize straight, hard fingers that are designed to push through strands of hair to the scalp. Although finger electrodes can be effective at reaching the scalp through many different hair types, they have several drawbacks. If the diameter of the finger is small enough to easily penetrate between hairs, it can become painful due to the high stress concentration where the sharp points contact the scalp. Larger diameter fingers that do not cause discomfort on the head often cannot penetrate between hairs without manual adjustment. Straight-finger electrodes, when made out of a hard material, are also an injury hazard under impact conditions. Examples of prior art dry electrodes are described in U.S. Pat. No. 4,967,038 and United States Patent Application Publication No. 2009/0030298 A1, and on the website of g.tec medical engineering under the heading: “g.SAHARA ACTIVE DRY EEG ELECTRODE SYSTEM”. 
Both wet electrodes and dry electrodes require some pressure application mechanism to physically secure them to the subject's head. For wet electrodes, the holding mechanism can be an adhesive, wherein the electrode is glued to the scalp. For dry electrodes, the holding mechanism includes elastic caps and mechanical headgear apparatus in the absence of an adhesive.